In Release-7 of Universal Mobile Telecommunications System (UMTS), the Single Cell Downlink Machine Input Machine Output (SC-MIMO) feature was introduced. SC-MIMO allows a Node-B to transmit two transport blocks to a single wireless transmit/receive unit (WTRU) from the same sector on a pair of transmit antennas improving data rates at high geometries and providing a beam forming advantage to the WTRU in low geometry conditions.
In Release-8 and Release-9 of UMTS, the Dual Cell High Speed Downlink Packet Access (DC-HSDPA) and Dual Band DC-HSDPA features were introduced. Both these features allow the Node-B to serve one or more users by simultaneous operation of HSDPA on two different frequency channels in the same sector, improving experience across the entire cell. The common part of these features is that they allow for simultaneous downlink reception of two independent transport blocks at the WTRU, where the transport blocks are transmitted on the High Speed Downlink Shared Channel (HS-DSCH) by a single Node-B sector.
Another technique that is based on the simultaneous reception of two or more transport blocks from different cells in the same or different frequency is multipoint operation. Multipoint operation consists in transmitting two independent transport blocks to the WTRU, wherein the transport blocks are transmitted from different Node-B sectors or cells on the same frequency or different frequency and geographically separated points. This may be seen as an extension of the DC-HSDPA feature on geographically separated cells on the same or different frequencies.
Multipoint transmissions may operate with two cells located on two different Node-Bs or in two different sites, referred to hereafter as inter-site mutilflow operation, and an Radio Network Controller (RNC) will split data between two Node-Bs. Each Node-B may then perform MAC and PHY layer operations, such as segmentation and TSN generation on the packets before transmission to the WTRU. Existing MAC and PHY layer procedures at the WTRU will not be able to process and reconstruct packets in sequence since they are originating from two Node-Bs. The MAC DC-HSDPA architecture in the WTRU is not designed to receive data from different sites. Additionally, reception from different sites may increase and introduce the possibility of out-of-order reception, causing potential data dropping in the MAC-ehs entities and premature RLC status reporting in the RLC entities. Methods to allow inter-site operation and reordering in several layers are required.